In phonetics, aspiration is the strong burst of air that accompanies either the release or, in the case of preaspiration, the closure of some obstruents. To feel or see the difference between aspirated and unaspirated sounds, one can put a hand or a lit candle in front of one's mouth, and say pin ([pʰɪn]) and then bin ([bɪn]). One should either feel a puff of air or see a flicker of the candle flame with pin that one does not get with bin. In most dialects of English, the initial consonant is aspirated in pin and unaspirated in bin.

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Voiceless consonants are produced with the vocal cords open and voiced consonants are produced when the vocal folds are fractionally closed. Voiceless aspiration occurs when the vocal cords remain open after a consonant is released. An easy way to measure this is by noting the consonant's voice onset time, as the voicing of a following vowel cannot begin until the vocal cords close.

Englishvoicelessstop consonants are aspirated for most native speakers when they are word-initial or begin a stressed syllable, as in pen, ten, Ken. They are unaspirated for almost all speakers when immediately following word-initial s, as in spun, stun, skunk. After s elsewhere in a word they are normally unaspirated as well, except when the cluster is heteromorphemic and the stop belongs to an unbound morpheme; compare dis[t]end vs. dis[tʰ]aste. Word-final voiceless stops optionally aspirate.

Aspirated consonants are not always followed by vowels or other voiced sounds; indeed, in Eastern Armenian, aspiration is contrastive even at the ends of words. For example compare: bard͡zpillow, with bart͡s⁼difficult and bart͡sʰhigh.

Alemannic German dialects have unaspirated [p⁼ t⁼ k⁼] as well as aspirated [pʰ tʰ kʰ]; the latter series are usually viewed as consonant clusters. In Danish and most southern varieties of German, the "lenis" consonants transcribed for historical reasons as ⟨b d ɡ⟩ are distinguished from their "fortis" counterparts ⟨p t k⟩ mainly in their lack of aspiration.

Icelandic and Faroese have pre-aspirated[ʰp ʰt ʰk]; some scholars interpret these as consonant clusters as well. Preaspirated stops also occur in some Sami languages; e.g. in Skolt Sami the unvoiced stop phonemes p, t, c, k are pronounced preaspirated (ʰp, ʰtʰcʰk) when they occur in medial or final position.

There are degrees of aspiration. Armenian and Cantonese have aspiration that lasts about as long as English aspirated stops, as well as unaspirated stops like Spanish. Korean has lightly aspirated stops that fall between the Armenian and Cantonese unaspirated and aspirated stops, as well as strongly aspirated stops whose aspiration lasts longer than that of Armenian or Cantonese. (See voice onset time.) An old IPA symbol for light aspiration was [ ʻ ] (that is, like a rotated ejective symbol), but this is no longer commonly used. There is no specific symbol for strong aspiration, but [ʰ] can be iconically doubled for, say, Korean *[kʻ ] vs. *[kʰʰ]. Note however that Korean is nearly universally transcribed as [k] vs. [kʰ], with the details of voice onset time given numerically.

Aspiration also varies with place of articulation. Spanish /p t k/, for example, have voice onset times (VOTs) of about 5, 10, and 30 milliseconds, whereas English /p t k/ have VOTs of about 60, 70, and 80 ms. Korean has been measured at 20, 25, and 50 ms for /p t k/ and 90, 95, and 125 for /pʰ tʰ kʰ/[citation needed].

Although most aspirated obstruents in the world's language are aspirated stops or aspirated affricates, aspirated fricatives such as [sʰ] have also been documented in some languages.

The word 'aspiration' and the aspiration symbol is sometimes used with voiced stops, such as [dʰ]. However, such "voiced aspiration", also known as breathy voice or murmur, is less ambiguously transcribed with dedicated diacritics, either [d̤] or [dʱ]. (Some linguists restrict the subscript diacritic [◌̤] to sonorants, such as vowels and nasal consonants, which are murmured throughout their duration, and use the superscript [◌ʱ] for the murmured release of obstruents.) When murmur is included under the term aspiration, as is common in Indo-Aryan linguistics, "voiceless aspiration" is called just that to avoid ambiguity.